Solid zirconium-based cross-linking agent and use in oil field applications

ABSTRACT

A solid zirconium cross-linking agent and use in a cross-linking composition in oil field applications such as hydraulic fracturing and plugging of permeable zones. The zirconium cross-linking agent is prepared by a process comprising contacting a zirconium complex with an alkanolamine and water at particular mole ratios of alkanolamine and water to zirconium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 11/986,397, filed Nov. 21, 2007 and published as U.S. Application Publication No. 2009/0131282 A1, entitled “Solid Zirconium-Based Cross-Linking Agent and Use in Oil Field Applications,” which is hereby incorporated herein by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to solid zirconium chelates and their use in oil field applications such as hydraulic fracturing and plugging of permeable zones.

BACKGROUND OF THE INVENTION

The production of oil and natural gas from an underground well (subterranean formation) can be stimulated by a technique called hydraulic fracturing, in which a viscous fluid composition (fracturing fluid) containing a suspended proppant (e.g., sand, bauxite) is introduced into an oil or gas well via a conduit, such as tubing or casing, at a flow rate and a pressure which create, reopen and/or extend a fracture into the oil- or gas-containing formation. The proppant is carried into the fracture by the fluid composition and prevents closure of the formation after pressure is released. Leak-off of the fluid composition into the formation is limited by the fluid viscosity of the composition. Fluid viscosity also permits suspension of the proppant in the composition during the fracturing operation. Cross-linking agents, such as borates, titanates or zirconates, are usually incorporated into the fluid composition to control viscosity.

Typically, less than one third of available oil is extracted from a well after it has been fractured before production rates decrease to a point at which recovery becomes uneconomical. Enhanced recovery of oil from such subterranean formations frequently involves attempting to displace the remaining crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) provide a bank of oil of substantial depth being driven into a producing well; however, in practice this is frequently not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling frequently occurs, so that the driving fluid flows preferentially through permeable zones depleted of oil (so-called “thief zones”) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable.

Difficulties in oil recovery due to thief zones may be corrected by injecting an aqueous solution of an organic polymer and a cross-linking agent into a subterranean formation under conditions where the polymer will be cross-linked to produce a gel, thus reducing permeability of the subterranean formation to the driving fluid (gas, water, etc.). Polysaccharide- or partially hydrolyzed polyacrylamide-based fluids cross-linked with certain aluminum, titanium, zirconium, and boron based compounds are used in these enhanced oil recovery applications. Cross-linked fluids or gels, whether for fracturing a subterranean formation or for reducing permeability of zones in subterranean formation, are now being used in hotter and deeper wells under a variety of temperature and pH conditions. In these operations the rate of cross-linking is critical to the successful generation of viscosity.

Oil field service companies are currently using zirconium or titanium based cross-linkers to generate viscosity in polysaccharide-based fluids useful in hydraulic fracturing, completion and enhanced oil recovery applications. Commercially available, zirconium-based cross-linkers containing an alkanolamine or a hydroxyalkylated ethylenediamine as a chelating ligand cross-link in the desired time and generate and maintain significant viscosity.

For example, U.S. Pat. No. 4,883,605 discloses a water-soluble zirconium chelate formed from a tetraalkyl zirconate and hydroxyethyl-tris-(2-hydroxypropyl) ethylenediamine, and the use of the chelate as a cross-linking agent in hydraulic fracturing fluids and in gels that are used for selectively plugging permeable zones in subterranean formations or for plugging subterranean leaks. Co-pending patent application, “Stable Solutions of Zirconium Hydroxyalkylethylene Diamine Complex and Use in Oil Field Applications”, U.S. patent application Ser. No. 11/643,513, filed Dec. 21, 2006, published at US 2008/0149341 A1, discloses a related complex having a 1:1 molar ratio of zirconium and N,N,N′,N′-tetrakis-(2-hydroxypropyl)-ethylenediamine.

These and other existing titanium or zirconium-based cross-linkers such as TYZOR TE organic titanate or TYZOR TEAZ organic zirconate are liquid products which either freeze or become too viscous to pump under cold temperature conditions, e.g., those encountered in underground wells in Canada and the Rocky Mountains. The need exists for solid cross-linkers which can be used to generate high, thermally stable viscosity in a low and/or high pH environment, and which can be used under these cold temperature conditions by pre-blending with the organic polymer, added as a solid to an aqueous polymer solution or dissolved in water and added to the aqueous polymer solution.

The need also exists for solid cross-linkers in off-shore fracturing operations, where the weight of chemicals being shipped and stored is critical. Solid cross-linkers, which contain two or more times the active Ti or Zr content of their liquid counterparts are desired because they would allow fracturing operations to be completed more economically.

The need also exists for solid cross-linkers that are non-flammable. Existing liquid solvent-based zirconate cross-linkers are flammable liquids. Even aqueous-based liquids generally comprise an organic co-solvent, and therefore are also flammable.

The present invention addresses these needs.

SUMMARY OF THE INVENTION

The present invention overcomes problems with liquid cross-linking agents by providing a process to prepare a solid cross-linking agent. The solid cross-linking agent is produced by a process which comprises: (a) contacting a zirconium complex with an alkanolamine in an alcohol solvent, wherein the mole ratio of alkanolamine to zirconium is 1:1; (b) adding water to the product of step (a) in an amount of 0.5 moles of water per mole of zirconium; (c) maintaining the product of step (b) for a sufficient period of time for the product to reach equilibrium; and (d) removing the solvent from the product of step (c) to form a solid zirconium cross-linking agent. In step (c), the temperature is generally from about 25° C. to about 90° C., preferably from 50° C. to 80° C. In step (d), the solvent is preferably removed by evaporation, for example, on a rotary evaporator. The temperature for evaporation should be below the melting or softening point of the zirconium cross-linking agent, preferably between 50° C. and 80° C.

There is further provided a cross-linking composition comprising (a) an aqueous liquid; (b) a buffer; (c) a cross-linkable organic polymer; and (d) a zirconium cross-linking agent, wherein the cross-linking agent is prepared by process comprising (1) contacting a zirconium complex with an alkanolamine in an alcohol solvent, wherein the mole ratio of zirconium to alkanolamine is 1:1; (2) adding water to the product of step (1) in an amount of 0.5 moles of water per mole of zirconium; (3) maintaining the product of step (2) for a sufficient period of time for the product to reach equilibrium; and (4) removing the solvent from the product of step (3) to form a solid zirconium cross-linking agent.

There is further provided methods for using the solid zirconium cross-linking agent in a cross-linking composition in oil field applications. Thus, there is provided a method for hydraulically fracturing a subterranean formation comprising introducing into a subterranean formation at a flow rate and pressure sufficient to create, reopen and/or extend a fracture in the formation, (a) an aqueous liquid; (b) a buffer; (c) a cross-linkable organic polymer; and (d) a zirconium cross-linking agent, wherein the cross-linking agent is prepared by process comprising (1) contacting a zirconium complex with an alkanolamine in an alcohol solvent, wherein the mole ratio of alkanolamine to zirconium is 1:1; (2) adding water to the product of step (1) in an amount of 0.5 moles of water per mole of zirconium; (3) maintaining the product of step (2) for a sufficient period of time for the product to reach equilibrium; and (4) removing the solvent from the product of step (3) to form a solid zirconium cross-linking agent. The components of the cross-linking composition can be introduced into the formation in any order or combination.

Optionally, the zirconium cross-linking agent may be dissolved in a solvent or solution (preferably water) containing optional components to provide a zirconium solution. The cross-linkable organic polymer is also preferably contacted with the aqueous liquid to prepare a base gel. The zirconium solution is introduced into the wellbore simultaneously with or sequentially after introducing the base gel into the wellbore. From the wellbore, the combination of base gel and zirconium solution are introduced into the formation.

This invention provides a method for plugging a permeable zone or leak in a subterranean formation which comprises introducing into said zone or said leak, (a) an aqueous liquid; (b) a buffer; (c) a cross-linkable organic polymer; and (d) a zirconium cross-linking agent, wherein the cross-linking agent is prepared by process comprising (1) contacting a zirconium complex with an alkanolamine in an alcohol solvent, wherein the mole ratio of alkanolamine to zirconium is 1:1; (2) adding water to the product of step (1) in an amount of 0.5 moles of water per mole of zirconium; (3) maintaining the product of step (2) for a sufficient period of time for the product to reach equilibrium; and (4) removing the solvent from the product of step (3) to form a solid zirconium cross-linking agent.

DETAILED DESCRIPTION OF THE INVENTION

Trademarks and tradenames are shown herein in upper case.

This invention is a process which provides a solid zirconium alkanolamine cross-linking agent at a particular mole ratio of alkanolamine to zirconium and at a particular mole ratio of water to zirconium. Surprisingly, it has been found that solid zirconium alkanolamine cross-linking agent is prepared according to the process of this invention that can be easily handled and overcomes the disadvantages of known zirconium alkanolamine cross-linking agents that are used as solutions.

The solid cross-linking agent is produced by a process which comprises: (a) contacting a zirconium complex with an alkanolamine at a particular mole ratio of alkanolamine to zirconium in an alcohol solvent; (b) hydrolyzing the product of step (a) by adding water; (c) maintaining the product of step (b) for a sufficient period of time for the product to reach equilibrium; and (d) removing the solvent from the product of step (c) to form a solid zirconium cross-linking agent. Preferably, water is added in an amount of about 0.5 mole of water for each mole of zirconium. Optionally, a diluent is added in step (c) or (d) to act as an anti-caking agent or to aid in flowability of the solid product.

While not wishing to be bound by theory, it is believed that the solid zirconium cross-linking agent produced comprises a mu oxo dimer of zirconium with alkanolamine. A mu oxo dimer is a combination of two zirconium atoms bridged by an oxygen atom. The structure or structures of such suspected mu oxo dimer has not been determined. The solid cross-linking agent may comprise other products, many of which will depend on reaction conditions.

A cross-linking composition based on the solid cross-linking agent prepared in this invention can be prepared by (1) blending the cross-linking agent with a cross-linkable organic polymer, then adding the blend as a solid to an aqueous liquid; or (2) dissolving the cross-linking agent in water, and adding the solution thus prepared to an aqueous liquid. In the latter alternative, the polymer may be pre-dissolved in the aqueous liquid or may be added to the aqueous liquid after the cross-linking agent has been added.

A solid zirconium cross-linking agent is prepared by a process which comprises contacting a zirconium complex with an alkanolamine in an alcohol solvent. The mole ratio of alkanolamine to zirconium is 1:1. Surprisingly, outside of these ranges, for example, at a molar ratio of 2 moles of alkanolamine to zirconium, a solid product does not form.

A number of tetraalkyl zirconates (also known as zirconium tetraalkoxides) can be used as the zirconium complex to prepare the above zirconium cross-linking composition, e.g., tetra-isopropyl zirconate, tetra-n-propyl zirconate, and tetra-n-butyl zirconate. The preferred tetraalkyl zirconate is tetra-n-propyl zirconate, available as TYZOR NPZ organic zirconate, a solution in n-propanol, with a zirconium content as ZrO₂ of about 28% by weight, and available from E. I. du Pont de Nemours and Company, Wilmington, Del.

Examples of suitable alkanolamines include, but are not limited to, triethanolamine, tri-n-propanolamine, tri-isopropanolamine, and diisopropanolamine. Preferably, the alkanolamine is triethanolamine.

Contacting the above tetraalkyl zirconates with the alkanolamine can be carried out at a variety of temperatures, e.g., between 25° C. and 90° C., preferably between 50° C. and 80° C., and in any order. The mixture is then maintained at this temperature for a sufficient period to reach equilibrium. For example, about 2 hours at 60° C. is adequate, but other periods may also be used.

The product of step (a) is then reacted with water in step (b) at a temperature between 25° C. and 90° C., preferably between 50° C. and 80° C., to form a hydrolysate. The amount of water should preferably be 0.5 mole of water per mole of zirconium to maximize the yield of desirable product, including the suspected mu oxo dimer of zirconium and alkanolamine. Surprisingly, outside of this ratio, for example, at a molar ratio of 2 moles of water per mole of zirconium, a solid product does not form.

Preferably, in step (b), water is added slowly as a dilute solution in an alcohol to minimize localized overreaction areas. For ease of alcohol recovery, it is preferred that the same alcohol be used as present in the zirconium complex. For example, if the zirconium complex is TYZOR NPZ organic zirconate, a dilute solution of water in n-propanol is used. By “dilute solution” is meant a solution of water in alcohol up to about 10% water in the alcohol. A solution of about 9-10% water in the alcohol is adequate dilution, but other concentrations may also be adequate.

In step (c), the product of step (b) is maintained at conditions for a sufficient period of time for the product to reach equilibrium. Equilibrium is reached after (1) the zirconium complex produced in step (a) has reacted with the water to form an intermediate product and (2) this intermediate product reacts with more product of step (a), as is proposed above, to form a mu-oxo dimer product. Time will depend on reaction conditions, such as temperature, pressure, mass transfer rates (e.g., depending on intensity of agitation). Thus, the process does not proceed to step (d) until the equilibrium is established. This step (c) can be carried out at a variety of temperatures, e.g., between 25° C. and 90° C., preferably between 50° C. and 80° C., and is then generally held at this temperature for a sufficient period to reach equilibrium. It has been found that holding the composition in step (c) at a temperature of 60° C. for about 6 hours is adequate, but other periods and temperatures may also be used.

In step (d), solvent is removed from the product of step (c), for example by distilling under vacuum, to remove the alcohol. Preferably, the distillation is carried out in a rotary evaporator, graining bowl, or other vacuum device with rotating parts to keep the solid in motion as it solidifies to limit and/or prevent lump formation. Spray drying may also be used in step (d) to remove the solvent. Many types of suitable equipment may be used, as are well known to those skilled in the art. The solution may also be simply dried in air, although for environmental reasons this is not a method of choice. The temperature of drying should be below the melting or softening point of the zirconium complex. Preferably, the temperature is between 50° C. and 80° C.

Preferably, a diluent is added to the solid cross-linking agent. A diluent may be added to the product of step (c) or to the solid after step (d). The diluent can be any material which may enhance properties of the solid, and does not adversely affect the solid or its use as a crosslinking agent. A diluent is added for such purposes as to help in the crystallization step, increase the crystallinity of the solid particles, provide an anti-caking action, improve the flowability of the solid material, enable easier solution of the solids in water, improve product stability, or for other purposes.

For example, the diluent may be a salt or metal oxide. The diluent may be selected from the group consisting of potassium chloride, tetramethylammonium hydroxide, titanium dioxide and silicon dioxide. Potassium chloride and tetramethylammonium hydroxide, for example, make the solid easier to remove from the drying unit than undiluted solids. The resulting solids are also more free-flowing and hence much more easily handled in the field. Insoluble inert compounds such as titanium dioxide and silicon dioxide may also be used to improve handling, and may be added before or after the drying step (d).

The solid zirconium cross-linking agent has a high concentration of zirconium relative to conventional cross-linking agents which are generally provided as solutions in water, alcohol or mixtures thereof. The zirconium content may be, for example, from 15 to 40% by weight based on the total weight of the cross-linking agent. Typical solutions of cross-linking agents comprise less than 10% zirconium by weight based on the total weight of the cross-linking agent.

The present invention also provides a cross-linking composition which comprises (a) an aqueous liquid; (b) a pH buffer; (c) a cross-linkable organic polymer; and (d) a zirconium cross-linking agent as prepared according to the process described above. Optionally, the composition may comprise a solvent.

The aqueous liquid (a) is typically selected from the group consisting of water, aqueous alcohol, and aqueous solution of a clay stabilizer. The alcohol is methanol or ethanol. Clay stabilizers include, for example, hydrochloric acid and chloride salts, such as, tetramethylammonium chloride (TMAC) or potassium chloride. Aqueous solutions comprising clay stabilizers may comprise, for example, 0.05 to 0.5 weight % of the stabilizer, based on the total weight of the cross-linking composition. Preferably, the clay stabilizer is tetramethylammonium chloride or potassium chloride. Preferably, the aqueous liquid is water, aqueous methanol, aqueous ethanol, an aqueous solution of potassium chloride, or a combination of two or more thereof.

The cross-linking composition comprises an effective amount of a pH buffer (b) to control pH. The pH buffer may be acidic, neutral or basic. The pH buffer is generally capable of controlling the pH from about pH 3 to about pH 12. For example, in a composition for use at pH of about 4-5, an acetic acid-based buffer can be used. In a composition for use at a pH of 5-7, a fumaric acid-based buffer or a sodium diacetate-based buffer can be used. In a composition for use at a pH of 7-8.5, a sodium bicarbonate-based buffer can be used. In a composition for use at a pH of 9-12, a sodium carbonate or sodium hydroxide-based buffer can be used. Other suitable pH buffers can be used, as are known to those skilled in the art.

The composition further comprises a cross-linkable organic polymer (c). Suitable cross-linkable organic polymers are selected from the group consisting of solvatable polysaccharides, polyacrylamides and polymethacrylamides. Preferably, the organic polymer is a solvatable polysaccharide and is selected from the group consisting of gums, gum derivatives and cellulose derivatives. Gums include guar gum and locust bean gum, as well as other galactomannan and glucomannan gums, such as those derived from sennas, Brazilwood, tera, honey locust, karaya gum and the like. Gum derivatives include hydroxyethylguar (HEG), hydroxypropylguar (HPG), carboxyethylhydroxyethylguar (CEHEG), carboxymethylhydroxypropylguar (CMHPG), carboxymethyl guar (CMG), and the like. Cellulose derivatives include those containing carboxyl groups, such as carboxymethylcellulose (CMC), carboxymethylhydroxy-ethylcellulose (CMHEC), and the like. The solvatable polysaccharides can be used individually or in combination; usually, however, a single material is used. Guar derivatives and cellulose derivatives are preferred, such as, HPG, CMC and CMHPG. HPG is generally more preferred based upon its commercial availability and desirable properties. However, CMC and CMHPG may be more preferred in cross-linking compositions when the pH of the composition is less than 6.0 or higher than 9.0, or when the permeability of the formation is such that one wishes to keep the residual solids at a low level to prevent damage to the formation.

The zirconium cross-linking agent (d) is the solid zirconium product prepared as described previously, and made by a method comprising: (a) contacting a zirconium complex with an alkanolamine in an alcohol solvent, wherein the mole ratio of alkanolamine to zirconium is 1:1; (b) adding water to the product of step (a) in an amount of 0.5 moles of water per mole of zirconium; (c) maintaining the product of step (b) for a sufficient period of time for the product to reach equilibrium; and (d) removing the solvent from the product of step (c) to form a solid zirconium cross-linking agent.

The zirconium cross-linking agent of the invention can be added as a solid or optionally dissolved in solution (preferably water). The solid cross-linking agent can be pre-blended with the solid polymer prior to dissolving both the solid cross-linking agent and solid polymer in the aqueous liquid. Alternatively, the solid zirconium cross-linking agent can be added to the aqueous liquid as a solid or in solution, before or after addition of the polymer. Typically, the cross-linkable polymer is mixed with the aqueous liquid such as water or mixed water/organic solvent or with an aqueous solution to form a base gel prior to adding the zirconium cross-linking agent. Organic solvents that may be used include alcohols, glycols, polyols, and hydrocarbons such as diesel.

The composition may comprise optional components, including those which are common additives for oil field applications. Thus, the composition may further comprise one or more of proppants, friction reducers, bactericides, hydrocarbons, chemical breakers, polymer stabilizers, surfactants, formation control agents, and the like. Proppants include sand, bauxite, glass beads, nylon pellets, aluminum pellets and similar materials. Friction reducers include polyacrylamides. Hydrocarbons include diesel oil. Chemical breakers break the cross-linked polymer (gel) in a controlled manner and include enzymes, alkali metal persulfate, and ammonium persulfate. Polymer stabilizers include methanol, alkali metal thiosulfate, and ammonium thiosulfate.

These optional components are added in an effective amount sufficient to achieve the desired cross-linking performance based on the individual components, desired cross-linking time, temperature and other conditions present in the formation being fractured or permeable zone being plugged.

The cross-linking composition is produced by mixing the solid or solution of zirconium cross-linking agent with the other components, in any order. A solution of zirconium cross-linking agent (zirconium solution) can be prepared by dissolving the solid zirconium cross-linking agent in an appropriate solvent or solution containing other components. Suitable solvents include water, water-alcohol mixtures, and alcohol wherein alcohol is methanol or ethanol. Preferably, the solvent is water. A solution containing other components may be an aqueous solution or an alcohol solution. Other components may include, for example, buffer, clay stabilizers, other optional components.

For example, in one particular application in an oil field, an aqueous solution of the solid zirconium cross-linking agent and optional components are introduced into a formation, while the cross-linkable organic polymer and aqueous liquid are introduced into the formation as a combined separate stream. Alternatively, all components may be premixed and introduced into a subterranean formation as a single stream. Advantageously, the components may be mixed in different combinations, and more advantageously, the components may be mixed just prior to use to enable easy variation and adjustment of the cross-linking rate.

This invention provides a method for hydraulically fracturing a subterranean formation, which comprises introducing into the formation at a flow rate and pressure sufficient to create, reopen, and/or extend one or more fractures in the formation, a cross-linking composition which comprises (a) an aqueous liquid; (b) a pH buffer; (c) a cross-linkable organic polymer; and (d) a solid zirconium cross-linking agent wherein the cross-linking agent is prepared by a process comprising: (1) contacting a zirconium complex with an alkanolamine in an alcohol solvent, wherein the mole ratio of alkanolamine to zirconium is 1:1; (2) adding water to the product of step (1) in an amount of 0.5 moles of water per mole of zirconium; (3) maintaining the product of step (2) for a sufficient period of time for the product to reach equilibrium; and (4) removing the solvent from the product of step (3) to form a solid zirconium cross-linking agent.

In one embodiment of the method for hydraulically fracturing a subterranean formation, a solution of solid zirconium cross-linking agent and cross-linkable polymer are contacted prior to their introduction into the formation, such that the cross-linking agent and polymer react to form a cross-linked gel. The cross-linked gel is then introduced into the formation at a flow rate and pressure sufficient to create, reopen, and/or extend a fracture in the formation. In this method, a base gel is prepared by mixing a cross-linkable organic polymer with an aqueous liquid. A cross-linked gel is prepared by mixing the base gel with a solution of the solid zirconium cross-linking agent, wherein the cross-linking agent is prepared by a process comprising: (a) contacting a zirconium complex with an alkanolamine in an alcohol solvent, wherein the mole ratio of alkanolamine to zirconium is 1:1; (b) adding water to the product of step (a) in an amount of 0.5 moles of water per mole of zirconium; (c) maintaining the product of step (b) for a sufficient period of time for the product to reach equilibrium; and (d) removing the solvent from the product of step (c) to form a solid zirconium cross-linking agent. The solution of the solid zirconium cross-linking agent is preferably prepared by dissolving the solid agent in water. At least one of the solution of zirconium cross-linking agent or the base gel further comprises a pH buffer.

Alternatively, the subterranean formation may be penetrated by a wellbore, such that contacting the solution of the solid zirconium cross-linking agent with the base gel occurs in the wellbore and the cross-linked gel is introduced into the formation from the wellbore. This method of hydraulically fracturing a subterranean formation penetrated by a wellbore comprises (a) preparing a base gel by mixing a cross-linkable organic polymer with an aqueous liquid; (c) introducing the base gel into the wellbore; (d) simultaneously with or sequentially after introducing the base gel into the wellbore, introducing a solution of the solid zirconium cross-linking agent; (e) permitting the base gel and the solution of zirconium cross-linking agent to react to form a cross-linked gel; and (f) introducing the cross-linked gel into the formation from the wellbore at a flow rate and pressure sufficient to create, reopen, and/or extend a fracture in the formation. A pH buffer is independently admixed with the base gel, the zirconium or both prior to introducing the base gel and the zirconium solution into the wellbore.

Upon creation of a fracture or fractures, the method may further comprise introducing a cross-linking composition comprising the zirconium solution, a cross-linkable organic polymer and proppant into the fracture or fractures. This second introduction of the zirconium solution is preferably performed in the event the cross-linking composition used to create the fracture or fractures did not comprise proppant.

Another use for the zirconium cross-linking agent of the present invention relates to a method for selectively plugging permeable zones and leaks in subterranean formations, which comprises introducing into the permeable zone or the site of the subterranean leak, a cross-linking composition comprising (a) an aqueous liquid; (b) a pH buffer; (c) a cross-linkable organic polymer; and (d) an aqueous solution of a solid zirconium cross-linking agent as described previously. The pH buffer may be admixed with the solution of zirconium cross-linking agent prior to introducing the cross-linking composition into the permeable zone or site of the leak.

In a first embodiment of the method for plugging a permeable zone or a leak in a subterranean formation, the aqueous liquid, pH buffer, cross-linkable organic polymer and the zirconium solution are contacted prior to their introduction into the subterranean formation, such that the polymer and zirconium cross-linking agent react to form a cross-linked aqueous gel, which gel is then introduced into the formation.

In an alternative embodiment of the method for plugging a permeable zone or a leak in a subterranean formation, the zirconium solution and the cross-linkable organic polymer are introduced separately, either simultaneously or sequentially, into the permeable zone or the site of the subterranean leak such that cross-linking occurs within the subterranean formation. This method comprises (a) preparing a base gel by mixing a cross-linkable organic polymer with an aqueous liquid; (b) introducing the base gel into the into the permeable zone or the site of the subterranean leak, (d) simultaneously with or sequentially after, introducing the base gel into the into the permeable zone or the site of the subterranean leak, introducing the zirconium solution into the permeable zone or the site of the subterranean leak; (e) permitting the base gel and the cross-linking agent to react to form a cross-linked aqueous gel to plug the zone and/or leak. At least one of the solution of zirconium cross-linking agent or the base gel further comprises a pH buffer.

The relative amounts of cross-linkable organic polymer and the zirconium complex may vary. One uses small but effective amounts which for both will vary with the conditions, e.g., the type of subterranean formation, the depth at which the method (e.g., fluid fracturing, permeable zone plugging or leak plugging) is to be performed, temperature, pH, etc. Generally, one uses as small an amount of each component as will provide the viscosity level necessary to effect the desired result, i.e., fracturing of the subterranean formation, or plugging permeable zones or leaks to the extent necessary to promote adequate recovery of oil or gas from the formation.

For example, satisfactory gels can generally be made for fluid fracturing by using the cross-linkable organic polymer in amounts up to about 1.2 weight % and the cross-linking composition in amounts up to about 0.50 weight % of the zirconium cross-linking agent, with percentages being based on the total weight. Preferably, from about 0.25 to about 0.75 weight % of the cross-linkable organic polymer is used and from about 0.05 to about 0.25 weight % of the zirconium cross-linking agent is used.

In a method for plugging permeable zones or leaks, generally about 0.25 to 1.2 weight % of a cross-linkable organic polymer is used, preferably 0.40 to 0.75 weight %, based on the total weight. Generally, about 0.01 to 0.50 weight % of the zirconium cross-linking agent is used, preferably 0.05 to 0.25 weight %, based on the total weight.

The amount of zirconium cross-linking agent used to cross-link the organic polymer is that which provides a zirconium ion concentration in a range from about 0.0005 weight % to about 0.1 weight %, based on the total weight. The preferred concentration of zirconium ion is in the range of from about 0.001-0.05 weight %, based on the total weight.

Typically, the solution of zirconium cross-linking agent of this invention can be used at a pH of from about 3 to 11. For low temperature applications (150-250° F., 66-121° C.), carbon dioxide-based energized fluids may be used. In this case, a pH for the cross-linking composition of about 3 to about 6 is preferred. For moderate or high temperature applications (250-400° F., 121-204° C.), a pH of about 9 to about 11 is preferred. Advantageously, the solution of zirconium complex of this invention is used at a temperature of 250-300° F. (121-149° C.).

EXAMPLES

The preparation of the compositions in the Examples and in the Controls were each carried out in closed vessels containing an agitator, thermometer, condenser, nitrogen inlet and dropping funnel. Unless specified otherwise, percentages are given by weight. Temperatures are given in degrees Celsius. The cross-linking properties of the compositions of this invention are given in the Examples as a function of the viscosity of carboxymethylhydroxypropylguar cross-linked with the zirconate of this invention.

Preparation of Base Gel

A Waring blender jar was filled with 1 liter of distilled water. To this was added 2 g of a 50% aqueous solution of tetramethylammonium chloride clay stabilizer. Agitation was started and 3.6 g of carboxymethylhydroxypropylguar (CMHPG) was sprinkled into the vortex of the agitating solution. The pH of the resultant slurry was adjusted to 6 with sodium diacetate and agitation continued for 30 minutes. The pH was then adjusted to 10.3 with 10% sodium hydroxide solution. Agitation was stopped and the gel was allowed to stand for 30 minutes or more before use.

Viscosity Measurement of Zirconate Cross-Linked Base Gel

To 250 ml of a vigorously agitated sample of base gel in a Waring blender jar, was added 0.00032 moles of zirconium (0.2-1.0 ml depending on percent zirconium in the solid cross-linking agent—hereinafter referred to as the Standard Loading Density). Agitation was continued for about 15-180 seconds. A 25 ml sample of the cross-linker containing gel was placed in the cup of the FANN 50 Viscometer with an R-1, B-3 configuration and viscosity was measured at 275° F. (135° C.) and 122 rpm at 100 reciprocal seconds of shear.

Comparative Example A

Triethanolamine (135.2 g) was added to 100 g of tetra-n-propyl zirconate solution (TYZOR NPZ organic zirconate, available from E. I. du Pont de Nemours and Company, Wilmington, Del.). The reaction mixture was heated to 60° C. and held there for 4 hours. Upon completion of the reaction the resultant solution of tetra(triethanolamine) zirconate was concentrated on a rotary evaporator under reduced pressure to yield 155 g of a viscous yellow oil, which contained 13.2% zirconium. This material was placed in a 32° F. (0° C.) freezer, which resulted in the material setting up as an immobile gummy solid.

This example shows that simply removing solvents from a solvent-based zirconate complex does not yield a solid that can be readily and easily handled for measuring and/or metering into a cross-linking composition.

Comparative Example B

This Comparative Example illustrates that preparing a zirconium triethanolamine cross-linking agent, having a mole ratio of triethanolamine to zirconium of 2:1, according to the process of this invention does not produce a solid cross-linking agent.

A 500-ml flask equipped with agitator, reflux condenser, N₂ source and agitator was charged with 100 g (0.227 moles) of tetra-n-propyl zirconate solution (TYZOR NPZ organic zirconate). Agitation was started and 67.6 g (0.454 moles) of triethanolamine were added dropwise. The reaction mixture was heated to 60° C. and held at this temperature for 2 hours. Then, a mixture glycerol, 20.9 g (0.227 moles), and water, 4.1 g (0.227 moles) was added dropwise to the reaction mixture to provide a solution. The solution was held at 60° C. for an additional 2 hours, and then the solvent removed on a rotary evaporator to give 124.8 g of a viscous yellow oil.

Comparative Example C

This Comparative Example illustrates that preparing a zirconium triethanolamine cross-linking agent, having a mole ratio of triethanolamine to zirconium of 8:1, but without water, according to the process of this invention does not produce a solid cross-linking agent.

A 500-ml flask equipped with agitator, reflux condenser, N₂ source and agitator was charged with 100 g (0.227 moles) of tetra-n-propyl zirconate solution (TYZOR NPZ organic zirconate). Agitation was started and 270.3 g (1.816 moles) of triethanolamine were added dropwise. The reaction mixture was heated to 60° C. and held at this temperature for 2 hours. Then, the solvent removed on a rotary evaporator to give 324.6 g of a viscous yellow oil.

Comparative Example D

This Comparative Example illustrates that preparing a zirconium triethanolamine cross-linking agent, having a mole ratio of triethanolamine to zirconium of 5:1 and large volume of water, according to the process of this invention does not produce a solid cross-linking agent.

A 1000-ml flask equipped with agitator, reflux condenser, N₂ source and agitator was charged with 10.0 ml (0.0227 moles) of tetra-n-propyl zirconate solution (TYZOR NPZ organic zirconate). Agitation was started and 20.0 ml (0.122 moles) of triethanolamine were added dropwise. The reaction mixture was heated to 60° C. and held at this temperature for 2 hours. Then, a mixture of 30.0 ml of n-propanol and 540 ml of water was added. The solvent was removed on a rotary evaporator to give 24.2 g of a viscous yellow oil.

Comparative Examples A through D show that simply removing solvent from a solvent-based zirconium complex, which can be used as a cross-linking agent, as described in the patent literature, does not yield a solid that can be readily and easily handled for measuring and/or metering into a cross-linking composition. Comparative Example A was a gummy solid. Comparative Examples B through D were viscous oils.

Comparative Example E

This Comparative Example illustrates that preparing a zirconium triethanolamine cross-linking agent, having a mole ratio of triethanolamine to zirconium of 2:1, according to the process of this invention does not produce a solid cross-linking agent.

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleed and condenser was charged with 176.2 g (0.400 moles) of TYZOR NPZ organic zirconate. Agitation was started and 119.2 g (0.801 moles) of triethanolamine were added. The resultant solution was held at 60° C. for two hours and then a solution of 3.6 g (0.20 moles) of water in 80 ml of n-propanol was added dropwise. The resultant solution was held at 60° C. an additional 6 hours and then placed on a rotary evaporator and the solvent removed under vacuum to give a viscous yellow oil.

This example shows that use more than a 1:1 molar ratio of alkanolamine to zirconium does not give a solid product on evaporation of solvent.

Comparative Example F

This Comparative Example illustrates that preparing a zirconium triethanolamine cross-linking agent, having a mole ratio of triethanolamine to zirconium of 2:1, according to the process of this invention does not produce a solid cross-linking agent.

A 500-ml flask, equipped with a thermocouple, dropping funnel, N₂ bleed and condenser was charged with 176.2 g (0.4 moles) of TYZOR NPZ organic zirconate. Agitation was started and 59.6 g (0.400 moles) of triethanolamine were added. The resultant solution was held at 60° C. for two hours and then a solution of 7.2 g (0.40 moles) of water in 80 ml of n-propanol was added dropwise. The resultant solution was held at 60° C. an additional 6 hours and then placed on a rotary evaporator and the solvent removed under vacuum to give a viscous yellow oil.

This example shows that use of more than a 0.5:1 molar ratio of water to zirconium alkanolamine complex does not give a solid product on evaporation of solvent.

Example 1

A 1000-ml flask, equipped with a thermocouple, dropping funnel, N2 bleed and condenser was charged with 352.4 g of TYZOR NPZ organic zirconate. Agitation was started and 119.2 g of triethanolamine were added. The resultant solution was held at 60° C. for two hours and then a solution of 7.2 g of water in 80 ml of n-propanol was added dropwise. The resultant solution was held at 60° C. an additional 6 hours and then placed on a rotary evaporator and the solvent removed under vacuum to give 196 g of a pale yellow solid containing 37.2% Zr.

Example 2

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleed and condenser was charged with 352.4 g of TYZOR NPZ organic zirconate. Agitation was started and 152.8 g of tri-isopropanolamine were added. The resultant solution was held at 60° C. for two hours and then a solution of 7.2 g of water in 80 ml of n-propanol was added dropwise. The resultant solution was held at 60° C. an additional 6 hours and then placed on a rotary evaporator and the solvent removed under vacuum to give 230 g of a pale yellow solid containing 31.7% Zr.

Example 3

A 1000-ml flask, equipped with a thermocouple, dropping funnel, N2 bleed and condenser was charged with 176.2 g of TYZOR NPZ organic zirconate. Agitation was started and 59.6 g of triethanolamine were added. The resultant solution was held at 60° C. for two hours and then a solution of 3.9 g of water in 40 ml of n-propanol was added dropwise. The resulting solution was held at 60° C. an additional 4 hours. This resulting solution was transferred to a 1000-ml pear shaped flask, 49 g of potassium chloride added and the solvent removed under vacuum on a rotary evaporator to give 196 g of a pale yellow solid containing 18.6% Zr. The resultant solid was much more easily removed from the rotary evaporator flask and was more granular in nature, i.e., easier to handle.

Example 4

A 1000-ml flask, equipped with a thermocouple, dropping funnel, N2 bleed and condenser was charged with 176.2 g of TYZOR NPZ organic zirconate. Agitation was started and 59.6 g of triethanolamine were added. The resultant solution was held at 60° C. for two hours and then a solution of 3.9 g of water in 40 ml of n-propanol was added dropwise. The resultant solution was held at 60° C. an additional 4 hours. The resultant solution was transferred to a 1000-ml pear shaped flask, 49 g of tetramethylammonium chloride added and the solvent removed under vacuum on a rotary evaporator to give 196 g of a pale yellow solid containing 18.6% Zr. The resulting solid was much more easily removed from the rotary evaporator flask and was more granular in nature, i.e., easier to handle.

Results

Table 1 below shows the performance of a 30 lb/1000 gallon (3600 g/1000 liters) CMHPG gel cross-linked with the zirconium products of the Examples. In this table, “Fann Time Max, min.” means the time, in minutes, it takes to reach maximum viscosity. The viscosity at this maximum time is labeled “Cp @ Max.”, to indicate viscosity in centipoise (Cp) and the viscosity after 90 minutes at the test temperature of 275° F. (135° C.) is labeled “Cp @ 90 min.”. The solid zirconate cross-linking agent was added to the pre-formed polymer base gel as a solid. Comparative Example A was warmed to lower viscosity and added as a viscous liquid.

TABLE 1 Example Performance Alkanolamine:Zr, Water:Zr, Fann Time Cp @ Cp @ 90 Example % Zr Alkanolamine mole ratio mole ratio Max, min Max min. Comparative A 13.2 Triethanolamine 4 1.5 1125 680 1 37.2 Isopropanolamine 1 0.5 3.5 1325 530 2 31.7 Triethanolamine 1 0.5 3.0 820 120 3 18.6 Triethanolamine 1 0.5 3.0 1200 622 4 18.6 Triethanolamine 1 0.5 2.5 1400 528

Table 1 shows that commercially available, cross-linking agent used in Comparative Example A generates and maintains good viscosity. However, when placed in a freezer, the material became an immobile gum, which would be very difficult to handle in colder climates.

The solid cross-linking agents of Examples 1 and 2, generate excellent viscosity at a desirable rate of cross-linking and, because they are solids, do not lose effectiveness in colder climates.

The addition of a diluent, such as potassium chloride or tetramethylammonium chloride improved granularity and flowability of the solid zirconate cross-linking agents of Examples 3 and 4, while maintaining good viscosity performance. 

What is claimed is:
 1. A method for hydraulically fracturing a subterranean formation comprising introducing into a subterranean formation at a flow rate and pressure sufficient to create, reopen and/or extend a fracture in the formation, (a) an aqueous liquid; (b) a buffer; (c) a cross-linkable organic polymer; and (d) a solid zirconium cross-linking agent comprising a zirconium-alkanolamine complex which comprises a mu oxo dimer of zirconium with alkanolamine.
 2. The method of claim 1 wherein the aqueous liquid, the buffer, the cross-linkable organic polymer, and the solid zirconium cross-linking agent are premixed and introduced into the subterranean formation as a single stream.
 3. The method of claim 1 wherein the subterranean formation is penetrated by a wellbore; the cross-linkable organic polymer and aqueous liquid are contacted to form a base gel; the solid zirconium cross-linking agent is dissolved in a solvent or solution containing other components to provide a zirconium solution; and wherein the method comprises introducing the base gel into the wellbore and/or formation; introducing the zirconium solution into the wellbore and/or formation simultaneously with or sequentially after introducing the base gel into the wellbore and/or formation; and permitting the base gel and the zirconium solution to react to form a cross-linked gel in situ within the wellbore and/or formation.
 4. The method of claim 3 wherein the solid zirconium cross-linking agent is dissolved in water.
 5. The method of claim 3 further comprising introducing a second composition comprising a zirconium solution, a cross-linkable organic polymer, and a proppant into the wellbore and/or formation.
 6. The method of claim 3 wherein the zirconium solution, the base gel or both further comprise a proppant.
 7. The method of claim 3 wherein the solid-zirconium cross-linking agent is used at a pH of from about 3 to
 11. 8. The method of claim 1 wherein the subterranean formation has a temperature of from about 150° F. to about 250° F.
 9. The method of claim 8 further comprising introducing to the subterranean formation carbon dioxide-based energized fluid.
 10. The method of claim 3 wherein the solid zirconium cross linking agent is present in an amount sufficient to provide a zirconium ion concentration of from about 0.0005 wt. % to about 0.1 wt. % based on the total weight of the crosslinked gel.
 11. A method for plugging a permeable zone or leak in a subterranean formation comprising introducing into said zone or said leak, (a) an aqueous liquid; (b) a buffer; (c) a cross-linkable organic polymer; and (d) a solution of a solid cross-linking agent comprising a zirconium-alkanolamine complex, wherein a mole ratio of alkanolamine to zirconium in the solid cross-linking agent is 1:1 and wherein the zirconium-alkanolamine complex is a mu oxo dimer of zirconium with alkanolamine.
 12. The method of claim 11 wherein the aqueous liquid, pH buffer, cross-linkable organic polymer; and solid cross-linking agent comprising a zirconium-alkanolamine complex are contacted prior to their introduction a permeable zone or leak zone in the subterranean formation.
 13. The method of claim 11 wherein the subterranean formation is penetrated by a wellbore; the solid cross-linking agent comprising a zirconium-alkanolamine complex is dissolved in a solvent or solution containing other components to provide a zirconium solution; and contacting of the zirconium solution with the aqueous liquid and the cross-linkable organic polymer occurs in the wellbore and/or formation.
 14. The method of claim 13 wherein the solid cross-linking agent comprising a zirconium-alkanolamine complex is dissolved in water.
 15. The method of claim 13 wherein the zirconium solution and cross-linkable polymer are introduced sequentially to the wellbore and/or formation.
 16. The method of claim 13 wherein the zirconium solution and cross-linkable polymer are introduced simultaneously to the wellbore and/or formation.
 17. The method of claim 13 wherein the zirconium solution, the base gel or both further comprise a proppant.
 18. The method of claim 13 wherein the solid-zirconium cross-linking agent comprising a zirconium-alkanolamine complex is used at a pH of from about 3 to
 11. 19. The method of claim 13 wherein the subterranean formation has a temperature of from about 150° F. to about 250° F.
 20. The method of claim 1, wherein the solid zirconium cross-linking agent has a zirconium content of from 15% to 40% based on the total weight of the crosslinking agent.
 21. The method of claim 11, wherein the solid crosslinking agent has a zirconium content of from 15% to 40% based on the total weight of the crosslinking agent.
 22. A method for hydraulically fracturing a subterranean formation comprising introducing into a subterranean formation at a flow rate and pressure sufficient to create, reopen and/or extend a fracture in the formation, (a) an aqueous liquid; (b) a buffer; (c) a cross-linkable organic polymer; and (d) a solid zirconium cross-linking agent comprising a zirconium-alkanolamine complex, wherein the solid zirconium cross-linking agent comprises water, wherein the mole ratio of water to zirconium in the solid zirconium cross-linking agent is 0.5:1.
 23. A method for plugging a permeable zone or leak in a subterranean formation comprising introducing into said zone or said leak, (a) an aqueous liquid; (b) a buffer; (c) a cross-linkable organic polymer; and (d) a solution of a solid cross-linking agent comprising a zirconium-alkanolamine complex, wherein a mole ratio of alkanolamine to zirconium in the solid cross-linking agent is 1:1, wherein the solid cross-linking agent comprises water, and wherein the mole ratio of water to zirconium in the solid cross-linking agent is 0.5:1. 